Recent trends in the telecommunication industry have been towards optical wavelength multiplexing for increasing bandwidth, and towards miniaturization of components and modules for higher integration. The particular case of spectrometers, including micro-spectrometers, in this and other industries has not escaped these tendencies.
In spectroscopy applications in general, i.e. in applications where wavelength separation and/or combination are required, several types of spectrometers are available. They include grating-based spectrometers, scanning Fourier transform spectrometer, and dispersive Fourier transform spectrometers.
An example of a grating-based spectrometer is that of the USB2000 model manufactured by Ocean Optics Inc. of Dunedin, Fla. It uses standard bulk optics, including a bulk grating, mounted in a relatively small package that can interface with a computer. More advanced micro-spectrometers using gratings include those with gratings formed by a LIGA process (x-ray lithography and micro-electroplating), such a micro-spectrometer being described by P. Krippner et al. in Proc. SPIE Vol. 2783, pp. 277-282, 1996. These grating-based spectrometers require taxing fabrication processes (LIGA process) and/or precise assembly of several bulk optics components such as gratings, mirrors, lenses and beamsplitters/combiners. Further, increasing the resolution of this type of grating-based spectrometers typically involves reducing the width of the entrance aperture (and the width of the exit aperture when present) or, more generally, increasing the F/# of the spectrometer. This leads to a reduction of light gathering efficiency, also known as étendue, which in turn yields higher acquisition time and/or spectra with a relatively low signal to noise ratio.
On the other hand, scanning Fourier transform spectrometers usually have large étendue and provide high resolution spectra. However, such benefits come at the cost of having one or more scanning elements, i.e. moving parts, which is an undesirable feature in applications where ease of manufacturing, ruggedness and low maintenance are desirable. Additionally, sufficient scanning amplitude of the scanning elements is required to obtain good spectral resolution. An example of a scanning Fourier transform spectrometer is given by O. Manzardo et al. in Optics Letters, Vol. 29, No. 13, Pp. 1437-1439, 2004. There, a micro-electro-mechanical system (MEMS) is used to form a miniature lamellar grating interferometer. However, the limited displacement amplitude of the moving elements (approximately 100 μm) fails to provide good spectral resolution.
Wavelength dispersive Fourier transform spectrometers have been disclosed in, for example, U.S. Pat. No. 5,059,027 issued Oct. 22, 1991, incorporated herein by reference. There, a collimating means is used to illuminate a diffraction grating-based dispersive two-beam interferometer, which provides, for a given wavelength, two wavefronts at its output, the wavefronts generally being at an angle with each other. The interference pattern formed by the two wavefronts is detected and analyzed to provide the spectral signal of the input light beam.
Harlander et al., in Applied Optics, vol. 41, pp. 1343-1352, 2002 also discloses a wavelength dispersive compact Fourier transform spectrometer, which can have a large spectral resolution. The spectrometers of U.S. Pat. No. 5,059,027 and of Harlander et al. includes collimating optics and a beamsplitter/combiner together with diffraction gratings and prisms. These optical elements involve delicate alignment, increase manufacturing complexity and do not easily lend themselves to miniaturization.
Accordingly, it is noted that diffraction grating-based spectrometers with high resolution have poor étendue. Further, scanning Fourier transform spectrometers commonly include moving parts requiring relatively large displacement amplitude to obtain high spectral resolution. Still further, existing dispersive Fourier transform spectrometers require collimating optics with beamsplitters/combiners, which increase manufacturing complexity. Therefore, it is desirable to provide a spectrometer having large étendue and high resolution while including a minimum number of collimating optics and beamsplitters/combiners, and being free of moving parts. Yet still further, it is also desirable to provide a spectrometer having the above-mentioned characteristics in addition to having a small form factor.